U.S. patent number 6,404,786 [Application Number 09/543,136] was granted by the patent office on 2002-06-11 for laser beam generating apparatus.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Kenji Kondo, Michio Oka, Hiroyuki Wada.
United States Patent |
6,404,786 |
Kondo , et al. |
June 11, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Laser beam generating apparatus
Abstract
A laser beam in the ultraviolet region is generated at high
power for along period. A green laser beam generated by a laser
oscillator comes incident into a resonator from behind a first
curved mirror, and circulates there, reflected by each mirror. By
passing a barium borate crystal, it causes a secondary harmonic (a
laser beam in the ultraviolet region) to be generated, which is
taken out of the resonator via a second curved mirror. The beam
waist of the laser beam passing the barium borate crystal is set to
46 .mu.m, about double the conventional thickness, by adjusting the
distance between the first curved mirror and a second flat mirror.
As a result, the power density of the laser beam in the barium
borate crystal is reduced to 1/4 of the value according to the
related art, and it is made possible to avoid rapid degradation of
the barium borate crystal by excessive squeezing of the laser
beam.
Inventors: |
Kondo; Kenji (Saitama,
JP), Oka; Michio (Tokyo, JP), Wada;
Hiroyuki (Kanagawa, JP) |
Assignee: |
Sony Corporation (Tokyo,
JP)
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Family
ID: |
27331365 |
Appl.
No.: |
09/543,136 |
Filed: |
April 5, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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136072 |
Aug 18, 1998 |
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Foreign Application Priority Data
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Aug 25, 1997 [JP] |
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9-228107 |
Sep 9, 1997 [JP] |
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9-243739 |
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Current U.S.
Class: |
372/22; 372/101;
372/92; 372/98; 372/95; 372/21 |
Current CPC
Class: |
G02F
1/3501 (20130101); G02F 1/3505 (20210101) |
Current International
Class: |
G02F
1/35 (20060101); H01S 3/02 (20060101); H01S
3/04 (20060101); H01S 3/081 (20060101); H01S
3/109 (20060101); H01S 5/022 (20060101); H01S
5/00 (20060101); H01S 003/10 (); H01S 003/08 () |
Field of
Search: |
;372/22,98,99,103,92,21,95,101 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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60-576695 |
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Apr 1985 |
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JP |
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4-84481 |
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Mar 1992 |
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JP |
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5-110174 |
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Apr 1993 |
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JP |
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Other References
Oka et al, "All-Solid-State Continuous Wave 0.1-W Ultraviolet
Laser," Conference on Lasers and Electro-Optics, vol. 12, Paper
CWQ7, 1992, pp. 374-375. .
M. Oka & S. Kubota, "Second-Harmonic Generation Green Laser for
Higher-Density Optical Disks," Japanese Journal of Applied Physics,
vol. 31, No. 2, Feb. 1992, pp. 513518..
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Primary Examiner: Ip; Paul
Assistant Examiner: Rodriguez; Armando
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Parent Case Text
PRIORITY
This application is a continuation-in-part application claiming the
benefit of priority to U.S. patent application Ser. No. 09/136,072,
filed on Aug. 18, 1998 abandoned.
Claims
What is claimed is:
1. A laser beam generating apparatus, wherein an ultraviolet beam
of not more than 400 nm in wavelength is generated by wavelength
conversion with a nonlinear optical crystal disposed in an external
resonator, and wherein 99.9% or more of the ambiance of its mirror
section and nonlinear optical crystal section is nitrogen.
2. A laser beam generating apparatus, wherein an ultraviolet beam
of not more than 400 nm in wavelength is generated by wavelength
conversion with a nonlinear optical crystal disposed in an external
resonator, and wherein the ambiance of its mirror section and
nonlinear optical crystal section is a gas whose moisture content
is not more than 0.1%.
3. A laser beam generating apparatus, wherein an ultraviolet beam
of not more than 400 nm in wavelength is generated by wavelength
conversion with a nonlinear optical crystal disposed in an external
resonator, and wherein the ambiance of its mirror section and
nonlinear optical crystal section is a gas whose hydrocarbon
content is not more than 0.1%.
4. A laser beam generating apparatus, wherein an ultraviolet beam
of not more than 400 nm in wavelength is generated by wavelength
conversion with nonlinear optical crystal disposed in an external
resonator, and wherein the ambiance of its mirror section and
nonlinear optical crystal section is a gas whose moisture content
and hydrocarbon compound content is not more than 0.1% and with an
oxygen content of 1% or more.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser beam generating apparatus,
and more particularly to a laser beam generating apparatus wherein
a resonator, provided outside of a laser oscillator, contains a
barium borate crystal and a laser beam in the ultraviolet region is
supplied, with harmonic content extracted from the laser beam
generated by the laser oscillator. In further detail, the invention
relates to an optical system for irradiating optical components
with an ultraviolet beam of not more than 400 nm in wavelength or a
laser beam generating apparatus for generating an ultraviolet beam
of not more than 400 nm in wavelength.
2. Description of the Related Art
If, in the field of semiconductor manufacturing for example, a
laser beam in the ultraviolet region can be used in a stepper (a
sequentially shifting exposure system), finer processing than what
is currently done will be made possible, enabling large-capacity
memory elements which are further enhanced in the level of
integration to be manufactured. A laser beam in the ultraviolet
region can be applied not only for this purpose but also to
photochemical reactions and biotechnology, and therefore practical
availability of ultraviolet lasers in many different fields is
awaited.
By a method according to the related art with high potential for
practical application to generate a laser beam in the ultraviolet
region, a barium borate crystal, which is a nonlinear optical
crystal, is disposed in a resonator provided outside the laser
oscillator, and secondary harmonic content is extracted from the
laser beam generator by the laser oscillator.
Where a laser beam in the ultraviolet region is to be generated by
this method, a harmonic content of the required intensity, i.e. an
ultraviolet laser beam, is obtained by squeezing the waist of the
laser beam (i.e. the radius of the cross section of the beam) which
is allowed to pass the barium borate crystal, because the nonlinear
conversion coefficient of the barium borate crystal is generally
small.
However, the squeezing of the waist of the laser beam results in a
greater power density of the laser beam in the barium borate
crystal, leading to the problem that they may be heavily damaged
both on the surface and inside.
Therefore, such a laser beam generating apparatus according to the
related art, although an ultraviolet laser beam is obtained in a
high output during the early phase of its use, steeply drops in
output with the lapse of time, making it difficult for a high
output to be maintained for a long period.
Incidentally, by the conventional method, if the power of an
ultraviolet laser beam is 100 mW, the output can last for not more
than 400 hours, and the velocity of degradation (the velocity of
power drop) is about 1.35.times.10.sup.-4 [%/hour].
The damage to the barium borate crystal can be more clearly
observed by microscope. FIG.
FIG. 1 is a schematic diagram showing the result of microscopic
observation of the trace of a beam pattern formed in a barium
borate crystal where the beam waist is 23 .mu.m.
This diagram is a front view of the laser beam emitting end face of
the barium borate crystal, in which the area surrounded by a dotted
line 102 is the part damaged by the laser beam, looking more turbid
than the surrounding normal part. Incidentally, it is because the
generated harmonic content spreads at an angle of about 4.degree.
to the original laser beam that the damage is oblong laterally.
Furthermore, there is another problem that optical components
deteriorate in performance characteristics when irradiated in the
atmosphere with an ultraviolet ray of not more than 400 nm in
wavelength, presumably because the optical losses of the optical
components increase in such a situation. Such optical losses are
presumed to occur as moisture and oily contents in the atmosphere
on the surface of the optical components react and the reaction
products and particles around them stick to the surface of the
optical components.
When an ultraviolet beam of not more than 400 nm in wavelength is
to be generated, in wavelength conversion using an external
resonator (for information on which, see M. Oka and S. Kubota, Jpn.
J. AppI. Phys. Vol. 31 (1992), pp. 513, and M. Oka et. al., in the
Digest of Conference on Laser and Electro-Optics (OSA, Washington,
D.C., 1992), paper CWQ7) or the like, the harmonic output is
significantly reduced by intricate performance deterioration of a
mirror or a nonlinear optical element arranged within the external
resonator. This deterioration again, as the present inventor sees
it, seems attributable to similar circumstances to what was
described above. When, for instance, an ultraviolet beam of not
more than 400 nm in wavelength formed by wavelength conversion
passes an optical component, such as a mirror, it adversely affects
the performance of the optical component (e.g. the mirror).
Therefore, for use where optical components are to be irradiated
with an ultraviolet beam of not more than 400 nm in wavelength as
well as where an ultraviolet beam of not more than 400 nm is to be
generated, there is a keen call for the development of an optical
system which can prevent the optical performance of optical
components from being adversely affected by an increase in optical
losses or their output performance and other attributes from being
deteriorated.
Problems with the aforementioned related art will be described
below with reference to drawings. For instance, where a dominant
wave of 532 nm in wavelength is to be converted in wavelength into
an ultraviolet beam of 266 nm in wavelength by using an external
resonator, the structure of the external resonator--art will be as
illustrated in FIG. 2.
In FIG. 2, what are denoted by reference numerals 10, 12 and 14 are
highly reflective mirrors having an ultra-high reflectance at a
wave-Length of 532 nm, e.g. a reflectance of 99. 95% or more; what
is denoted by numeral 8 is an incidence mirror having a high
reflectance, e.g. a reflectance of 99% at a, wavelength of 532 nm;
and what is denoted by numeral 6 is a nonlinear optical crystal
BBO, which is a wavelength converting element coated with a less
reflective film having a low reflectance, e.g. a reflectance of not
more than 0.1% at a wavelength of 532 nm., The highly reflective
mirror 14 is installed over a VCM (see the above-cited SRF92
collection of preliminary papers), which is a positioning device
(not shown), and can be controlled by, for instance, a servo drive
system. The elements 6, B, 10, 12 and 24 referred to above
constitute an external resonator section.
When a dominant Wave (of 532 nm in wavelength here) schematically
indicated by an arrow 30 in FIG. 2 is brought to incidence on this
external resonator, it is amplified between the mirrors, and the
amplified dominant wave is converted by the nonlinear optical
crystal 6 (BBO) into a secondary harmonic (of 266 nm in wavelength
here). This secondary harmonic is schematically indicated by an
arrow 31 in FIG. 2.
When such a wavelength conversion as described above is
accomplished in the atmosphere, optical losses (to be specific,
mainly scattering) of the mirrors (especially the mirror 10)
increase. The relationship between an optical loss and the power of
the dominant wave of 532 nm in wavelength, amplified in the
external resonator, can be represented by the following
equation.
Where .delta.cav is the optical loss at a wavelength of 532=in the
external resonator; P.omega., the power of the amplified dominant
wave; Pi, the power of the dominant wave of 532 nm in wavelength
coming incident on the external resonator; and .gamma..sub.SH, a
constant known as a nonlinear conversion factor determined by the
crystalline length of the nonlinear optical crystal 6 (BBO),
wavelength of the dominant wave, spot size and focusing
parameter.
Equation 1 given above reveals that, in the external resonator, the
power P.omega. of the dominant wave decreases with an increase in
the optical loss .delta.cav.
On the other hand, the relationship between the power of the
dominant wave and that of the secondary harmonic can be represented
by Equation 2 below.
Where P.omega. is the power of the dominant wave coming incident on
the nonlinear optical crystal 6 (BBO); P.sub.2.omega. the power of
the secondary harmonic generated by wavelength conversion by the
nonlinear optical crystal 6 (BBO); and .gamma..sub.SH, said
nonlinear conversion factor.
Equation 2 given above reveals that, when the power P.omega. of the
dominant wave decreases, the power P.sub.2.omega. of the secondary
harmonic also decreases. In a rough measure, the power of the
secondary harmonic halves in about 5 to 10 hours.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a laser beam
generating apparatus capable of generating a laser beam in the
ultraviolet region at a high power level for a long period of
time.
In order to achieve the above-stated object, according to the
invention, there is provided a laser beam generating apparatus
comprising a laser oscillator; an external resonator on which a
laser beam emitted from said laser oscillator comes incident; and a
barium borate crystal disposed on an optical path within said
external resonator, whereby a harmonic content is extracted from
said laser beam emitted from said laser oscillator to supply a
laser beam in the ultraviolet region, having a configuration in
which the length of said barium borate crystal along said optical
path is within the range of 2 mm to 6 mm and the beam waist of said
laser beam passing said barium borate crystal in the position of
said barium borate crystal is within the range of 40 .mu.m to 60
.mu.m.
According to the invention, there is also provided a laser beam
generating apparatus comprising a laser oscillator; an external
resonator on which a laser beam emitted from said laser oscillator
comes incident; and a barium borate crystal disposed on an optical
path within said external resonator, whereby a harmonic content is
extracted from said laser beam emitted from said laser oscillator
to supply a laser beam in the ultraviolet region, having a
configuration in which the length of said barium borate crystal
along said optical path is greater than 6 mm and the beam waist of
said laser beam passing said barium borate crystal in the position
of said barium borate crystal is greater than 60 .mu.m.
A laser beam generating apparatus according to the invention,
having a configuration in which the length of the barium borate
crystal along the optical path is within the range of 2 mm to 6 mm
and the beam waist of said laser beam passing said barium borate
crystal is within the range of 40 .mu.m to 60 .mu.m, and the power
density of the laser beam in the barium borate crystal is thereby
prevented from becoming greater than necessary, is increased in the
service life of the barium borate crystal and enabled to generate
an ultraviolet laser beam for a long period of time at a high
output.
Further, a laser beam generating apparatus according to the
invention, having a configuration in which the length of the barium
borate crystal along the optical path is greater than 6 mm and the
beam waist of the laser beam passing the barium borate crystal is
greater than 60 .mu.m, and the power density of the laser beam in
the barium borate crystal is thereby prevented from becoming
greater than necessary, is increased in the service life of the
barium borate crystal and enabled to generate an ultraviolet laser
beam for a long period of time at a high output.
In order to achieve the above-stated object, in an optical system
irradiated with an ultraviolet beam according to the invention,
optical components are irradiated with an ultraviolet beam of not
more than 400 nm in wavelength, and 99.9% or more of the ambience
of the optical components is nitrogen.
In another such optical system, 99.9% or more of the ambience of
the optical components is air.
In still another such optical system, the ambience of the optical
components is a gas whose moisture content is not more than
0.1%.
In yet another such optical system, the ambience of the optical
components is a gas whose hydrocarbon compound content is not more
than 0.1%.
In order to achieve the above-stated object, in a laser beam
generating apparatus according to the invention, where an
ultraviolet beam of not more than 400 nm in wavelength is to be
generated by wavelength conversion with a nonlinear optical crystal
disposed in an external resonator, 99.9% or more of the ambiance of
its mirror section and nonlinear optical crystal section is
nitrogen.
In another such laser beam generating apparatus, 99.9% or more of
the ambiance of its mirror section and nonlinear optical crystal
section is air.
In still another such laser beam generating apparatus, the ambiance
of its mirror section and nonlinear optical crystal section is a
gas whose moisture content is not more than 0.1%.
In yet another such laser beam generating apparatus, the ambiance
of its mirror section and nonlinear optical crystal section is a
gas whose hydrocarbon compound content is not more than 0.1%.
In another such laser beam generating apparatus, the ambiance of
its mirror section and nonlinear optical crystal section is a gas
whose moisture content and hydrocarbon compound content is not more
than 0.1%. At the same time, 1% or more of the ambiance of its
mirror section and nonlinear optical crystal is oxygen. The
above-mentioned value of ratios is based on volume percentage.
The present invention is a result of various studies taking note of
the ambiance in which irradiation with an ultraviolet beam of not
more than 400 nm in wavelength is done, or such an ultraviolet beam
is generated, an aspect which had not been considered previously,
especially with respect to the purity of nitrogen or air or the
proportion of the moisture or oily (hydrocarbon compound) content.
According to the invention, an ultraviolet beam of not more than
400 nm in wavelength can give a satisfactory result for the object
of the invention as long as the purity of nitrogen or air or the
proportion of the moisture or oily (hydrocarbon compound) content
is within the applicable range envisaged according to the
invention.
Incidentally, although the Gazette of the Japanese Patent Laid-open
No. Sho 60-57695 discloses a technique to seal in a laser element
airtightly and thereby prevent its deterioration, that of the
Japanese Patent Laid-open No. Hei 4-84481 discloses a technique to
protect a laser element by enclosing inert gas in the package of a
semiconductor laser apparatus, and that of the Japanese Patent
Laid-open No. Hei 5-110174 discloses a technique to use inert gas
as the ambiance of a laser diode, but none of these disclosures
concerns a configuration similar to the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing the result of microscopic
observation of the trace of a beam pattern formed in a barium
borate crystal where the beam waist is 23 .mu.m.
FIG. 2 illustrates the configuration of an external resonator for
use in wavelength: conversion to an ultraviolet beam of not more
than 400 nm in wavelength.
FIG. 3 illustrates the configuration of an essential part of a
laser beam generating apparatus according to the present
invention.
FIG. 4 is a graph representing the relationship between the
distance L between a first curved mirror and a first flat mirror
and the beam waist in the position of the barium borate
crystal.
FIG. 5 is a graph showing the service life of a resonator actually
determined.
FIG. 6 is a diagram illustrating the action of a first preferred
embodiment of the invention.
FIG. 7 is a graph showing problem of usual technology.
FIG. 8 is a graph showing one embodiment of the invention.
FIG. 9 shows plots of UV Power versus time for several moisture
contents of the ambiance around the optics of the laser of the
invention over an operating period of 20 hours.
FIG. 10 shows plots of UV Power versus time for several moisture
contents of the ambiance around the optics of the laser of the
invention over an operating period of 1000 hours.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Next, preferred embodiments of the present invention will be
described with reference to drawings.
FIG. 3 illustrates the configuration of an essential part of a
laser beam generating apparatus according to the present
invention.
As illustrated, a laser beam generating apparatus 2, which is a
preferred embodiment of the invention, comprises a laser oscillator
3, a resonator on which a laser beam generated by the laser
oscillator 3 comes incident, and a barium borate crystal 6 disposed
on an optical path within the resonator 4.
The resonator 4 further comprises first and second curved mirrors 8
and 10 disposed at an interval between and substantially opposite
to each other, and first and second flat mirrors 12 and 14 disposed
at an interval between and substantially opposite to each
other.
Each of the mirrors is arranged at one or another of the apexes of
an inverted trapezoid symmetric with respect to a vertical
centerline, with the first and second curved mirrors 8 and 10
positioned at the two ends of the longer of the two parallel sides
of said trapezoid, and the first and second flat mirrors 12 and 14,
positioned at the two ends of the shorter of the two parallel sides
of said trapezoid.
The first curved mirror 8 and the second flat mirror 14 are
disposed adjacent to each other, and the second curved mirror 10
and the first flat mirror 12 adjoin each other.
The distance between the first and second curved mirrors 8 and 10
is about 120 mm, and that between the first curved mirror 8 and the
first flat mirror 12 is 95.+-.10 mm, both in this particular
embodiment.
Each mirror is disposed with, an appropriate inclination with the
result that the laser beam reflected by the first curved mirror 8
comes incident on the second curved mirror 10, the laser beam
reflected by the second curved mirror 10 comes incident on the
second flat mirror 14, the laser beam reflected by the second flat
mirror 14 comes incident on the first flat mirror 12, and the laser
beam reflected by the first flat mirror 12 comes incident on the
first curved mirror 8.
The angle formed by the laser beam coming incident on the first
flat mirror 12 and that reflected by the first flat mirror 12 is
approximately 20.degree. in this particular embodiment.
In this embodiment, every one of the second curved mirror 10, and
the first and second flat mirrors 12 and 14 has a reflectance of no
less than 99.9%, and the first curved mirror 8 has a reflectance of
99%.
The barium borate crystal (BBO) 6 as a nonlinear optical crystal is
arranged between the first and second curved mirrors 8 and 10. The
laser beam emitted from the first curve mirror 8 comes incident on
the second curved mirror 10 via this barium borate crystal 6. The
length of the barium borate crystal 6 along the optical path is 6
mm in this particular embodiment.
On two end faces 16 of the barium borate crystal 6 respectively
facing the first and second curved mirrors 8 and 10 have
anti-reflection (AR) coats, so that the laser beam coming incident
on the barium borate crystal 6 and that emitted from the barium
borate crystal 6 pass these end faces 16 virtually without being
reflected. The residual reflectance is 0.03% in this particular
embodiment.
The laser oscillator 3 generates a green laser beam of 532 nm in
wavelength in this particular embodiment. The laser beam generated
by the laser oscillator 3 travels along, a straight line linking
the first and second curved mirrors 8 and 10, and comes incident
into the resonator 4 from behind the first curved mirror 8.
Near the rear side of the first curved mirror 8 is disposed a
photodetector 18, which can receive the laser beam emitted from the
laser oscillator 3 and coming incident on and reflected by the
first curved mirror 8.
The first flat mirror 12 is fitted to a voice coil motor 20 and, by
driving the first flat mirror 12 with this voice coil motor 20, the
distance between the first curved mirror 8 and the first flat
mirror 12 can be adjusted in an order of a few nm.
A servo control unit 22 constantly monitors the result of detection
by this photodetector 18, and controls the voice coil motor 20 so
as to minimize the laser beam intensity detected by the
photodetector 20, so that the green laser beam from the laser
oscillator 3 can efficiently come incident into the resonator
4.
The basic operation of this laser beam generating apparatus
proceeds as described below.
The green laser beam generated by the laser oscillator 3 comes
incident into the resonator 4 from behind the first curved mirror
8.
Then, the laser beam reflected toward the rear side of the first
curved mirror 8 is detected by the photodetector 18.
The servo control unit 22, as stated above, is constantly
monitoring the result of detection by the photodetector 18, and
controls the voice coil motor 20 so as to minimize the laser beam
intensity detected by the photodetector 20.
This enables the green laser beam from the laser oscillator 3 to
efficiently come incident into the resonator 4 and eventually a
high power ultraviolet laser beam to be provided.
The laser beam coming incident into the resonator 4 travels
straight ahead, passes the barium borate crystal 6, and comes
incident on the second curved mirror 10.
The laser beam coming incident on the second curved mirror 10 is
reflected by the second curved mirror 10 to come incident on the
second flat mirror 14.
The laser beam coming incident on the second flat mirror 14 is
reflected by the second flat mirror 14 to travel toward the first
flat mirror 12, comes incident on the first flat mirror 12, and is
reflected by the first flat mirror 12.
After that, the laser beam comes incident on the first curved
mirror 8, is reflected by the first curved mirror 8, and thereafter
circulates in the resonator 4 by the route hitherto described.
When the laser beam passes the barium borate crystal 6, the barium
borate crystal 6 generates the secondary harmonic of the original
green laser beam of 532 nm in wavelength, i.e. an ultraviolet laser
beam of 266 nm in wavelength, which is emitted from the resonator 4
via the second curved mirror 10.
Since the laser beam passing the barium borate crystal 6 is
squeezed into a thin beam, as described above, to facilitate the
generation of the secondary harmonic by the barium borate crystal
6, the laser beam is appropriately squeezed by the first and second
curved mirrors 8 and 10.
In this preferred embodiment, however, with a view to forestalling
the adverse effect of squeezing the laser beam excessively, the
waist of the laser beam (the radius of the beam cross section) in
the position of the barium borate crystal 6 is set to a proper
value by adjusting the distance between the first curved mirror 8
and the first flat mirror 12.
FIG. 4 is a graph representing the relationship between the
distance L between the first curved mirror 8 and the first flat
mirror 12 (therefore the distance between the second curved mirror
10 and the second flat mirror 14) and the beam waist in the
position of the barium borate crystal 6, wherein the calculated
values of the beam waist along with the variation of said distance
L are plotted.
As is seen from this graph, the beam waist gradually narrows with
an increase in the distance L and, as the distance L is varied from
85 mm to 105 mm, the beam waist varies from about 55 .mu.m to about
56 .mu.m.
In this laser beam generating apparatus 2 embodying the invention,
the beam waist is set to 46 .mu.m, about double the conventional
thickness, by adjusting the distance L. As a result, the power
density of the laser beam in the barium borate crystal 6 has been
reduced to 1/4 of the value according to the related art, and it is
made possible to avoid rapid damaging of the barium borate crystal
6 by excessive squeezing of the laser beam.
FIG. 5 is a graph showing the service life of the laser beam
generating apparatus actually determined. In the graph, the
horizontal axis represents time, and the vertical axis, the power
(in mW) of the laser beam.
Herein, a polygonal line 24 represents the power of the ultraviolet
laser beam emitted from the resonator 4; a polygonal line 26, the
power of the laser beam generated by the laser oscillator 3; and a
polygonal line 28, the power of the laser beam transmitted by the
first curved mirror 8 and coming incident into the resonator 4.
In experimentation, said power levels were measured while adjusting
the power of the laser beam generated by the laser oscillator 3 so
as to keep the power of the ultraviolet laser beam constantly at
100 mW.
Accordingly, while the power of the laser beam to be supplied to
the resonator 4 must steadily increase with the progress of damage
to the barium borate crystal 6, the power of the laser beam
supplied from the laser oscillator 3 to the resonator 4 scarcely
increased, as is seen from the graph of FIG. 5, even when the laser
beam generating apparatus 2 was operated for as long as 1200 hours.
That is to say, a long service life of 1200 hours has been
realized.
The rate of degradation is 6.9.times.10.sup.-5 [%/hour] according
to the result of this experiment, representing a significant
improvement over the conventional level of 1.35.times.10.sup.-4
[%/hour].
The ultimate service life of the apparatus can be estimated at
approximately 5000 hours from this rate of degradation.
Microscopic observation of the barium borate crystal 6 revealed no
trace of the beam pattern illustrated in FIG. 1.
An experiment, in which the length of the barium borate crystal 6
and the beam waste of the laser beam passing the barium borate
crystal 6 were varied in many different ways, high power laser
beams of 100 mW or more were obtained for a long period of no less
than 1000 hours if the beam waist of the laser beam passing the
barium borate crystal 6 was set in a range of 40 .mu.m to 60 .mu.m,
where the length of the barium borate crystal 6 along the optical
path was between 2 mm and 6 mm.
In a similar experiment, it was confirmed that, where the length of
the barium borate 6 along the optical path was longer than 6 mm,
high power laser beams of 100 MW or more could be obtained for a
long period of no less than 1000 hours if the beam waist of the
laser beam passing the barium borate crystal 6 was set wider than
60 .mu.m.
An optical system for irradiation with an ultraviolet beam
according to the invention is an optical system in which optical
components are irradiated with an ultraviolet beam of not more than
400 nm in wavelength, and 99.9% or more of the ambiance of the
optical components is nitrogen, or 99.9% or more of the ambiance of
the optical components is air, or the ambiance of the optical
components is a gas whose moisture content is not more than 0.1%,
or is a gas whose hydrocarbon compound content is not more than
0.1%, or the ambiance of optical component is a gas whose moisture
content and hydrocarbon compound content is not more than 0.1% and
an oxygen content of 1% or more.
In this case, means to seal in the optical components with any of
said ambient gases, i.e. one with a nitrogen content of 99.9% or
more, one with an air content of 99.9% or more, one whose moisture
content is not more than 0.1%, or one whose hydrocarbon compound
content is not more than 0.1%, or one whose moisture content and
hydrocarbon compound content is not more than 0.1% and with an
oxygen content of 1% or more can be used to place the optical
components in any of said ambiances.
Or else, means to purge the peripheries of the optical components
with any of said ambient gases can be used as well. Purging, for
example, can be accomplished by boring two or more holes in a
sealed container, through one of which any of the above-mentioned
ambient gases, i.e. one with a nitrogen content of 99.9% or more,
one with an air content of 99.9% or more, one whose moisture
content is not more than 0.1%, or one whose hydrocarbon compound
content is not more than 0.1%, or one whose moisture content and
hydrocarbon compound content is not more than 0.1% and with an
oxygen content of 1% or more is blown in and through the other of
which the gas originally present in the sealed container is
expelled to replace the latter with the ambient gas blown in.
Also, an optical system for irradiation with an ultraviolet beam
according to the invention may as well be an optical system in
which, where an ultraviolet beam of not more than 400 nm in
wavelength is to be generated by wavelength conversion with a
nonlinear optical crystal disposed in an external resonator, its
mirror section and nonlinear optical crystal section are placed in
an ambiance with a nitrogen content of 99.9% or more, or with an
air content of 99.9% or more, or in a gas whose moisture content is
not more than 0.1%, or a gas whose hydrocarbon compound content is
not more than 0.1%. In this case, means to seal in said mirror
section and nonlinear optical crystal section with any of said
ambient gases, i.e. one with a nitrogen content of 99.9% or more,
one with an air content of 99.9% or more, one whose moisture
content is not more than 0.1% or one whose hydrocarbon compound
content is not more than 0.1%, or one whose moisture content and
hydrocarbon compound content is not more than 0.1% and with an
oxygen content of 1% or more can be used to place these sections in
any of said ambiances.
Or, means to purge said sections with any of said ambient gases can
be used as well. Purging, for example, can be accomplished by
boring two or more holes in a sealed container in which to arrange
these sections, through one of which any of the above-mentioned
ambient gases, i.e. one with a nitrogen content of 99.9% or more,
one with an air content of 99.9% or more, one whose moisture
content is not more than 0.1%, or one whose hydrocarbon compound
content is not more than 0.1%, or one whose moisture content and
hydrocarbon compound content is not more than 0.1% and with an
oxygen content of 1% or more is blown in and through the other of
which the gas originally present in the sealed container is
expelled to replace the latter with the ambient gas blown in.
Specific preferred embodiments of the present invention will be
described below.
EMBODIMENT 1
This embodiment represents an application of the invention to an
optical system for ultraviolet beam generation having an external
resonator section, described with reference to FIG. 2, i.e. a
wavelength converting system wherein a nonlinear optical crystal is
disposed in an external resonator.
This embodiment is an optical system in which, where an ultraviolet
beam of not more than 400 nm in wavelength is to be generated by
wavelength conversion with a nonlinear optical crystal disposed in
an external resonator illustrated in FIG. 2, its mirror section and
nonlinear optical crystal section are placed in an ambiance with a
nitrogen content of 99.999% or more. The high reflection mirrors
10, 12 and 14, the incidence mirror 8, and the nonlinear optical
crystal (BBO) 6 in FIG. 2 are placed in this ambiance with a
nitrogen content of 99.999% or more. More specifically, the inside
of the external resonator shown in FIG. 2 was purged with nitrogen
of 99.999% in purity, and wavelength conversion was carried out in
a way similar to the above-described.
As a result, the optical losses of the mirrors did not increase,
and the power of the secondary harmonic remained without drop for
1000 hours or more, as shown in FIG. 6. FIG. 6, in which the
horizontal axis represents time (in hours) and the vertical axis,
the power (P.sub.2.omega./MW) I shows how the power of the
secondary harmonic behaves over time. As is seen from FIG. 6, the
output of the secondary harmonic remained free from attenuation for
1000 hours or more.
When wavelength conversion as described above was accomplished,
byway of comparison, under the same conditions as the
above-described except that the inside of the external resonator
was in natural atmosphere, the optical losses of the mirror
increased and the output of the secondary harmonic dropped, as
shown in FIG. 7. Data in FIG. 7 are smoothed by averaging. FIG. 7,
in which the horizontal axis represents time (in hour) and the
vertical axis, the ultraviolet power (in mW), shows how the power
of the second harmonic behaves over time. As is seen from FIG. 7,
the output of the second harmonic drops into 0 mW approximately 20
hours after.
EMBODIMENT 2
This embodiment, like Embodiment 1, represents an application of
the invention to an optical system for ultraviolet beam generation
having an external resonator section, described with reference to
FIG. 2, i.e. a wavelength converting system wherein a nonlinear
optical crystal is disposed in an external resonator. Herein, the
inside of the external resonator shown in FIG. 2 was purged with
air of 99.999% in purity, and wavelength conversion was carried out
in a way similar to the above-described.
As a result, a similar effect to that of Embodiment 1 was achieved.
Especially, UV power becomes stable, when 1% or more of the
ambiance of optical component is oxygen.
EMBODIMENT 3
This embodiment, like Embodiment 1, represents an application of
the invention to an optical system for ultraviolet beam generation
having an external resonator section, described with reference to
FIG. 2, i.e. a wavelength converting system wherein a nonlinear
optical crystal is disposed in an external resonator. Herein, the
inside of the external resonator shown in FIG. 2 was purged with a
gas with a moisture content of 0.001%, and wavelength conversion
was carried out in a way similar to the above-described.
As a result, a similar effect to that of Embodiment 1 was
achieved.
EMBODIMENT 4
This embodiment, like Embodiment 1, represents an application of
the invention to an optical system for ultraviolet beam generation
having an external resonator section, described with reference to
FIG. 2, i.e. a wavelength converting system wherein a nonlinear
optical crystal is disposed in an external resonator, wherein the
inside of the external resonator shown in FIG. 2 was purged with a
gas with a hydrocarbon content of 0.001%, and wavelength conversion
was carried out in a way similar to the above-described.
As a result, a similar effect to that of Embodiment 1 was
achieved.
EMBODIMENT 5
This embodiment, like Embodiment 1, represents an application of
the invention to an optical system for ultraviolet beam generation
having an external resonator section, described with reference to
FIG. 2, i.e. a wavelength converting system wherein a nonlinear
optical crystal is disposed in a external resonator. Herein, the
inside of the external resonator shown in FIG.2 was purged with
nitrogen of 99.9% in purity, and wavelength conversion was carried
out in a way similar to the above described.
As a result, power of the second harmonic as obtained for 1000
hours or more, although it slightly dropped, as shown in FIG. 8.
FIG. 8 in which the horizontal axis represents time (in hour) and
the vertical axis, the ultraviolet power (in mW), shows how the
power of the second harmonic behaves over time. Data in FIG. 8 are
smoothed by averaging. As :is seen from FIG. 8, the output of the
second harmonic was obtained for 1000 hours or more, although it
was obtained only for 20 hours or so with the external resonator in
natural atmosphere.
FIG. 9 shows plots of UV Power versus time for several moisture
contents of the ambiance around the optics of the laser of the
invention over an operating period of 20 hours. A comparison of the
plots shown in FIG. 9 illustrate how an ambient moisture content of
0.1% is very advantageous over a moisture content of 1.0%. For
example, a laser outputting 50 mW at time 0 outputs. 0 mW at 20
hours in an ambient having a moisture content of 1.0%, whereas the
same laser outputs substantially 50 mW at 20 hours in an ambient
having a moisture content of 0.1%.
FIG. 10 shows plots of UV Power versus time for several moisture
contents of the ambiance around the optics of the laser of the
invention over an operating period of 1000 hours. A comparison of
the plots shown in FIG. 10 illustrate how an ambient moisture
content of 0.1% is very advantageous over a moisture content of
1.0%, and how a moisture content of 0.001 is advantageous over
either of a moisture content of 1.0% or 0.1%. For example, a laser
outputting 50 mW at time 0 outputs 0 mW at 1000 hours in an ambient
having a moisture content of 1.0%, and the same laser outputs
around 12 mW at 1000 hours in an ambient having a moisture content
of 0.1%, whereas the same laser outputs substantially 50 mW at 1000
hours in an ambient having a moisture content of 0.001%.
EMBODIMENT 6
This embodiment, like Embodiment 1, represents an application of
the invention to an optical system for ultraviolet beam generation
having an external resonator section, described with reference to
FIG. 2, i.e. a wavelength converting system wherein a nonlinear
optical crystal is disposed in a external resonator. Herein, the
inside of the external resonator shown in FIG. 2 was purged with
air of 99.9% in purity, and wavelength conversion was carried out
in a way similar to the above described.
As a result, a similar effect to that of Embodiment 5 war,
achieved.
EMBODIMENT 7
This embodiment, like Embodiment 1, represents an application of
the invention to an optical system for ultraviolet beam generation
having an external resonator section, described with reference to
FIG. 2, i.e. a wavelength converting system wherein a nonlinear
optical crystal is disposed in a external resonator. Herein, the
inside of the external resonator shown in FIG. 2 was purged with a
gas with a moisture content of 0.1%, and wavelength conversion was
carried out in a way similar to the above described.
As a result, a similar effect to that of Embodiment 5 was
achieved.
EMBODIMENT 8
Embodiment 8, like Embodiment 1, represents an application of the
invention to an optical system for ultraviolet beam generation
having an external resonator section, described with reference to
FIG. 2, i.e. a wavelength converting system wherein a nonlinear
optical crystal is disposed in a external resonator. Herein, the
inside of the external resonator shown in FIG. 2 was purged with a
gas with a hydrocarbon content of 0.1%, and wavelength conversion
was carried out in a way similar to the above described.
As a result, a similar effect to that of Embodiment 6 was
achieved.
EMBODIMENTS 9 to 16
Whereas the external resonator sections of these embodiments had
respectively the same ambiances as those of Embodiments 1 to 8,
those of Embodiments 9 to 16 were sealed in from the outset with
the respective ambient gases instead of purging them with the
respective ambient gases as in Embodiments 1 to 8.
EMBODIMENTS 17 to 24
These embodiments are optical systems in which optical components
are irradiated with an ultraviolet beam of not more than 400 nm in
wavelength, wherein the peripheries of the optical components are
surrounded by the respectively similar ambiances to those in
Embodiments 1 to 8.
EMBODIMENTS 25 to 32
These embodiments are optical systems in which optical components
are irradiated with an ultraviolet beam of not more than 400 nm in
wavelength, wherein the peripheries of the optical components are
surrounded by the respectively similar ambiances to those in
Embodiments 9 to 16.
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